1. Technical Field
The present invention relates to an optical element including a multilayer film and a method of manufacturing the same, and, more particularly to a cross dichroic prism for color combination or color separation used in a projector apparatus and a method of manufacturing the same.
2. Related Art
A multi-plate projector often adopts a system for subjecting white light to color separation to illuminate valves for displaying images of respective colors and, after combining the images of the respective colors, projecting an image obtained by combining the images on a screen with a projection lens. A cross dichroic prism is one of optical products (optical components) suitable for separating white light into three colors and composing a three-color screen.
A structure of a cross dichroic prism in the past is shown in section in FIG. 22. In a cross dichroic prism 90 shown in the figure, dichroic films 92 made of multilayer films are formed on respective one surfaces of four prisms 91a to 91d of a triangular prism shape obtained by dividing the cross dichroic prism 90 into four pieces. The dichroic films 92 are bonded by an adhesive 93 having an optical characteristic substantially equivalent to that of a base material of the prisms. In the figure, the dichroic films 92 and the adhesive 93 are shown with sufficient thicknesses in order to illustrate presence thereof. However, as publicly known, actual thicknesses of the dichroic films 92 and the adhesive 93 are very small, for example, about several tens micrometers. The same holds true for other drawings attached to this specification.
FIG. 23 is another example of the cross dichroic prism. In a cross dichroic prism 95 shown in the figure, first, a pair of prisms divided into two that sandwich a dichroic film 92 made of a multilayer film are formed by four prisms 91a to 91d of a triangular prism shape obtained by dividing the cross dichroic prism 95 into four pieces. Thereafter, the dichroic film 92 made of a multilayer film is formed in a junction of one of the pair of prisms divided into two. Junctions of the pair of prisms divided into two are bonded by an optical adhesive 93.
In a cross dichroic prism, it is effective for, in particular, prevention of blurring of an image and double images to form an optical path to be optically uniform because the formation of the optically uniform optical path improves an image quality. For example, it is desirable to make refractive power of members constituting the optical path uniform. As disclosed in JP-A-06-331807, it is important for improvement of performance of the cross dichroic prism to highly accurately manage uniformity of refractive indexes of members constituting respective prisms. As disclosed in JP-A-09-015405, since a level difference occurs in the dichroic film 92 due to the thickness of the adhesive 93 in the constitution of the dichroic prism 90 in the past shown in FIG. 22, it is desirable to adopt the constitution shown in FIG. 23.
In both the constitutions, since a part of the optical path is formed by the adhesive 93, it is important for further improvement of performance of the dichroic prisms 90 and 95 to make an optical characteristic of the adhesive 93 identical with that of the prisms 91a to 91d. Therefore, an adhesive having a refractive index after hardening substantially the same as that of glass is developed as an optical adhesive.
However, although the adhesive is hardened by polymerization due to heat or polymerization due to light, it is difficult to stably and uniformly harden the adhesive so as to have a refractive index same as that of glass. For example, refractive power after hardening fluctuates more or less because of fluctuation in components of materials of the adhesive and fluctuation in hardening conditions. Moreover, although there are more than one type of glass adoptable as prisms, types of the glass adoptable as prisms are limited unless the adhesive corresponds to the types of the glass.